In a self-convergence type color cathode ray tube comprising an in-line type electron gun for aligning three electron beams and a deflection yoke for generating a horizontal deflection magnetic field distorted in a pincushion shape and a vertical deflection magnetic field distorted in a barrel shape, the distortion of the deflection magnetic fields for self-convergence causes a quadrupole astigmatism of the electron beams.
As a technology for correcting the astigmatism, Japanese Patent Application (Tokkai Sho) No. 61-99249 describes correcting the cross-sectional shape of the electron beams by producing a nonaxisymmetric lens between a first focusing electrode and a second focusing electrode according to the deflection degree.
FIG. 7 shows the electron gun of a color cathode ray tube apparatus according to this prior art. Vertically long electron beam openings 18 are provided on the face of a first focusing electrode 14 at the side facing a second focusing electrode 15. Horizontally long electron beam openings 19 are provided on the face of the second focusing electrode 15 at the side facing the first focusing electrode 14. A constant focusing voltage Vfoc is applied to the first focusing electrode 14. A dynamic focusing voltage that gradually increases from the focusing voltage Vfoc with the increase of the deflection amount is applied to the second focusing electrode 15. This technology can obtain a dynamic focusing action in response to a change in the electron beam focusing amount caused by a difference in the distance between the electrode gun and the phosphor screen when the electron beams are deflected and an action for correcting the astigmatism of the electron beams caused by the deflection magnetic field simultaneously.
However, the above prior art can only correct the same amount of astigmatism of the three electron beams simultaneously.
The actual astigmatism amounts of the right and left electron beams are different from each other. Furthermore, the difference in the astigmatism amount is opposite depending on the deflection derection.
FIG. 8 shows the size of forces acting on three electron beams emitted from an in-line type electron gun in the deflection magnetic field generated by a self-convergence type deflection yoke. As can be seen from FIG. 8, a force that distorts the cross-sectional shape of the electron beams to be horizontally long acts most strongly on the right electron beam (R beam) when the electron beams are deflected to the right side of the screen. The larger the deflection degree is, the stronger the force is. On the other hand, the force that distorts the cross-sectional shape of the electron beams to be horizontally long acts most strongly on the left electron beam (B beam) when the electron beams are deflected to the left side of the screen.
The difference in the astigmatism amount among the electron beams is significant especially in a color cathode ray tube having a large deflection angle and a short total length and a color cathode ray tube having a substantially flat phosphor screen, and prevents implementing a high resolution over the whole phosphor screen.